Directional microwave transmission system having dielectric lens



H. B. DEVORE DIRECTIONAL MICROWAVE TRANSMISSION SYSTEM HAVING DIELECTRIC LENS Aug. 23, 1949.

2 Sheets-Sheet 1 Filed Aug. 20, 1945 HTTO/P/VZY Aug. 23, 1949.

Filed Aug. 20, 1945 H. B. DEVORE DIRECTIONAL MICROWAVE TRANSMISSION SYSTEM HAVING DIELECTRIC LENS 2 Sheets-Sheet 2 MEOW/IVE SOURCE ZSnventor HenryBDeI bre C(ttorneg Patented Aug. 23, 1949 DIRECTIGNAL MICROWAVE TRANSMISSION SYSTEM HAVING DIELECTRIC LENS Henry B. De Vore, Cranbury, N. 3., assignor to Radio Corporation of America, a

of Delaware corporation Application August 20, 1945, Serial No. 611,649

This invention relates generally to microwave transmission and more particularly to a method of and means for proportioning a dielectric element disposed between conductive elements to minimize reflections of microwaves propagated be tween said conductive elements.

Frequently radiating systems for microwave electromagnetic radiation utilize solid dielectric elements disposed between metallic conductive elements. For example, the dielectric element may be in the form of a lens between parallel metal sheets employed to focus the microwave radiation, or the dielectric element may comprise a mechanical support for the metallic conducting members. It is often desirable that the microwave transmission characteristic be substantially uniform throughout the region occupied by the dielectric element.

In wave guides or microwave transmission systems containing parallel disposed conductive plates, non-uniform microwave transmission characteristics are produced by the existence of an air gap of varying thickness between the surface of the dielectric and the adjacent metal surface.

to the dielectric surface, such bonds are difficult to obtain and are not always feasible in the particular mechanical structure desired. However, it is customary to apply to the surface of the dielectric element facing the metal surface a thin metallic coating which provides for uniform transmission of the radiation in the region adjacent to the dielectric. A small portion of the incident radiant energy will be propagated through the air gap between the coated dielectric element and the surface of the metallic element. Microwave transmission through this air gap may cause interference with the microwaves propagated through the dielectric element, thereby pro:- viding undesirable reflections in the transmission system. Also it is essential that the dielectric element be so proportioned that internal reflections of incident microwaves therein substantially cancel external surface reflections therefrom.

Among the objects of the instant invention are to provide an improved method of and means for minimizing wave reflections in a microwave transmission system which. includes a solid dielectric element. proved method of and means for proportioning a solid dielectric element in a microwave guide for minimizing wave reflections and wave interference due to the presence of said dielectric element in said guide. An additional object of While this difficulty, in some instances, may be avoided by bonding the metal surface Another object is to provide an im- 2 Claims. (Cl. 178-44) the invention is to provide an improved microwave focusing element in a wave transmission system. Another object is to provide a substantially reflectionless solid dielectric element disposed within a microwave transmission guide. A further object is to provide an improved method of and means for proportioning a dielectric element in a'microwave guide system to minimize surface reflections from said dielectric element as well as wave interference between Waves propagated through said element and waves propagated around said element.

The invention will be described in greater detail by referring to the accompanying drawing of which Figure 1 is an elevational cross-sectional view of a first embodiment of the invention, Figure 2 is a plan cross-sectional view taken along the section line II-II of a second embodiment of the invention, Figure 3 is an elevational crosssectional View taken along the section line III-III of said second embodiment of the invention and Fig. 4 is a perspective view thereof. similar reference characters are applied to similar elements throughout the drawing.

Referring to the drawing, Figure 1 shows a wave guide I having microwaves propagated therethrough in the direction indicated by the arrow 3. A dielectric element 5, having metallic surface coatings l and 9 disposed on the surfaces thereof adjacent to the inner walls of the wave guide I, is proportioned to have a length L of.

a critical value for minimizing surface reflections of incident microwaves and interference between microwaves transmitted through the dielectric element and microwaves transmitted through the gap between the wave guide I and the conductive coatings I and 9.

It is known that the wave energy reflected by a dielectric element will be minimized if the thickness of the element in the direction of wave propagation is an integral number of half wavelengths of the electromagnetic radiation, as measured in the particular dielectric material. For a dielectric element of such length, the internal microwave radiation reflected by the back face I l re-emerges from the front face I3 in exactly opposite phase to the external radiation reflected by the front face l3. Thus the two wave reflection components produce destructive wave interference and tend to cancel, with the result that minimum radiation is reflected back toward the source of wave radiation.

It is also desirable to eliminate, insofar as possible, the disturbing effects caused by wave radiation which travels through the small wave gaps between the coated surfaces 1 and 9 of the dielectrio and the inner surface of the wave guide I. If the radiation travelling'through these air gaps does not emerge therefrom in phase with the wave radiation which travels directly through the dielectric element '5, destructive interference between the fields: of; said. radiation will occur, with the result; that thewaveradiation propagated beyond the dielectric element will be reduced in intensity and part of such radiation will be re fiected back toward the source. of, wage. energy In order to reduce such Ie'fieCtlI0lE-1S,'lhe; length, L of the dielectric element 5 is selected effectively to cancel internal and externaliwave reflections in the dielectric element as wel l 'as to provide a minimum of wave interference between waves propagated through the dielectric: element. and; waves propagated through the air gap between said element and the inner surface of the wave guide. If the dielectric element has a dielectric constan la. and the wave.- length; at the radiation 5 he: wa length; in the dielectric will be In ordeig to: satisfy the; condition. for minimum waverreilection from thedielectric' faces I l and tit, the leng'th. oi; the dielectric element shouldibe periods. In order to. avoid.v destructirze interference b ween radiation. travelling through: the air gaps. and. that travelling, through the dielectric: el ment, the difieren'cei in. propagation time should be an. integral. number of. periods, such that Ila/t L x T' "I where in is an integer.

The optimum condition. to, satisiy both Equations. (lyand (2). is realized combining. the

eqhatid s whereby where n. an m both are, integers. and. where. It also is. the. ratio: of. the. dielectric. constants or the two dielectrics, since the. dielectric constantofi is substantially unity. For. most. dielectric mate the, quantity.

, W i not. be accurately expressed as the ratio of two reasonably small integers, and hence both types, oi reflection phenomenamay not be: completelyeliminatedby utilizing dielectric elements. having reasonably short lengths; Howener, in:

'4 general, the radiation reflections from the dielectric faces are large compared with the reflections due to phase interference of waves propagated through the air gaps, providing the air gaps are held to reasonably small dimensions. Therefore, it is desirable to choose a length for the dielectric element. such; that. the condition represented by the. Equation (1).: is accurately satisfied and the condition represented by the Equation (2) is approximately satisfied.

As. an. illustration, the following calculations indicate the; shortest length L of a polystyrene dielectric element which will provide minimum reflection for radiation of 0.5 in wave length. Forth is radiation the dielectric constant of polystyrene is 2.56. Hence substituting in Equation (1.),

Hence it. is seen. that the optimum length for the polystyrene element. are those multiples of where.- M is the. wavelength in. air (or vacuum). In. g rder that. reflections fromv frontand back bloc}; surfaces cancel, since: there is a phase reuersal at one reflective'su-riace and not at the other; the. bloclg length must. be an integral number oil halt wavelengths of the radiation in the block),-. Hence:

2L /E 5; T \r z? )\o.

the gate, the number of periods required to propagatea distance L; is.

Y P. 7&1

where in is the wavelength in the, fluid; medium and lc gits dielectric constant. The number of periods requiredlto propagate through the blockis In order to have no interference, the difference of theseltwo' numbers of periods must be an, integer:

L. L. L T y 5 Ti, Vic-Film combining, both conditions: are satisfied:

auaova where k is the ratio of the dielectric constants.

Referring to Figures 2 and 3, the principles of the instant invention are applied to the design of a corrective dielectric lens employed in cooperation with a microwave reflector whereby undesirable reflections from the lens surfaces are minimized. While the thickness of the lens varies throughout its width, there is a comparatively wide region near the center of the lens throughout which the thickness is relatively constant. Since a proportionally large fraction of the radiation is directed through the central region of the lens, proportioning the lens thickness in the central region to values which approximately satisfy Equations (1) and (2) eliminates a large proportion of the undesirable wave reflections from the lens surfaces.

In Figures 2 and 3 a microwave generator 2| is coupled through a wave guide 23 to a point 25 at the focus of a toroidal wave reflector 21. The waves radiated by the wave guide 23 are guided to the reflector 21 between parallel disposed conductive plates 29 and 3|. The incident waves from the wave guide 23 pass through a dielectric corrective lens 33, are reflected by the metallic reflector 21 and pass to the output wave path 35, which is in a plane displaced with respect to the plane of the waveguide 23 and corrective lens 33. Preferably, the corrective lens 33 has conductive coatings 31 and 39 on the upper and lower surfaces thereof in close proximity to the inner surfaces to the conductive plates 29 and 3|, respectively, thereby providing air gaps 4| and 43 of the same type as described heretofore with respect to the device of Figure 1. Side conductive elements 45 and 41 are optional and are employed merely to prevent wave leakage between the edges of the conductive plates 29 and 3|, and to provide supporting elements for said plates.

It should be understood that the use of the conductive coatings on the surface on the dielectric element adjacent to the inner surface of the wave guiding means is optional. However, in general, the use of such conductive coatings on the dielectric element provides a more uniform leakage path through the air gap between the dielectric element and the inner surface of the wave guiding means, the reflection effects of which may be more effectively neutralized by proper proportioning of the length of the dielectric element.

Thus the invention disclosed comprises an improved method of and means for reducing reflections in a wave guiding system due to the presence of a, solid dielectric element by suitably proportioning the length of said dielectric element to neutralize surface reflections therefrom and efiectively to minimize wave interference between waves propagated through said element and waves propagated through the spaces surrounding said element.

I claim as my invention:

1. In a microwav dielectric guide having a first dielectric with a dielectric constant 701, a substantially reflectionless second dielectric lens element having a dielectric constant k2 disposed transversely within said guide, said element having a length 1m mk WE SW1; along the axis of wave propagation, where A0 is the wavelength in air or vacuum, and n and m are integers.

2. A device according to claim 1 including conductive means on the surfaces of said element adjacent to the inner surfaces of said guide.

HENRY B, DE VORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,129,669 Bowen Sept. 13, 1938 2,407,911 Tonks Sept. 17, 1946 2,408,271 Rigrod Sept. 24, 1946 

